A promising means for breaking the limit of scaling in semiconductor memories for large scale integrated circuits (LSIs), such as a static random access memory (SRAM) and a dynamic random access memory (DRAM), is replacement of an existing semiconductor memory with a nonvolatile memory. The proposed specific nonvolatile memories include a phase-change random access memory (PRAM), a resistive random access memory (ReRAM), and a magnetic random access memory (MRAM). In particular, the MRAM is a memory for storing information based on a magnetization direction of a magnetic substance. The MRAM is superior to other memories in terms of high-speed operation, and is therefore expected to be replace an existing work memory.
Although the MRAM has been put into practical use in a limited market, the primary challenge to be addressed for enlarging the market in the future is to reduce a write current. The reason is that a larger write current leads to an increased size of a cell transistor for driving a cell, with the result that the cell size becomes larger to increase bit cost. A large write current is not preferred also in terms of power consumption during operation.
A write method for the MRAM in practical use at present involves the use of the Oersted magnetic field, which is induced by a current introduced in wiring, to switch the magnetization of a memory element arranged in the vicinity of the Oersted magnetic field. This method, however, uses a current of several milliamperes (mA), and the write current becomes still larger when the design rule is smaller. This is not preferred in terms of application to a leading-edge CMOS generation.
A promising method for reducing the write current in the MRAM is a method using spin-transfer torque induced by a current, and has been actively studied and developed. The feature of the method using spin-transfer torque resides in introducing a current directly into a magnetic substance rather than introducing a current into metal wiring arranged in the vicinity of the magnetic substance. As the specific method, there have been proposed an MRAM element in which current-induced magnetization switching is used for writing and an MRAM element in which current-induced domain wall motion is used for writing. In both the MRAM elements, the write current is reduced along with miniaturization in element size. In other words, the write current is scalable. It is therefore expected that write operation be achieved with a remarkable small current as compared to the conventional method using the Oersted magnetic field. However, any of those methods is not different from the method using the Oersted magnetic field in terms of using a current for writing.
By the way, in recent years, it has been reported that magnetic properties of a ferromagnetic substance, such as the magnitudes of magnetic anisotropy and saturation magnetization, may be modulated with the use of a voltage instead of a magnetic field or a current (see Non Patent Literature 1 and Non Patent Literature 2). If a voltage can be used for writing instead of a current, power consumption in writing can be remarkably suppressed. An MRAM for performing writing with the use of a voltage is disclosed in Patent Literature 1.